Method for reducing harmonic distortion in comb drive devices

ABSTRACT

Methods of fabricating comb drive devices utilizing one or more sacrificial etch-buffers are disclosed. An illustrative fabrication method may include the steps of etching a pattern onto a wafer substrate defining one or more comb drive elements and sacrificial etch-buffers, liberating and removing one or more sacrificial etch-buffers prior to wafer bonding, bonding the etched wafer substrate to an underlying support substrate, and etching away the wafer substrate. In some embodiments, the sacrificial etch-buffers are removed after bonding the wafer to the support substrate. The sacrificial etch-buffers can be provided at one or more selective regions to provide greater uniformity in etch rate during etching. A comb drive device in accordance with an illustrative embodiment can include a number of interdigitated comb fingers each having a more uniform profile along their length and/or at their ends, producing less harmonic distortion during operation.

FIELD OF THE INVENTION

The present invention relates generally to the field of semiconductormanufacturing and microelectromechanical systems (MEMS). Morespecifically, the present invention pertains to fabrication methods forreducing harmonic distortion in comb drive devices.

BACKGROUND OF THE INVENTION

Electrostatic comb drive devices are utilized to provide movement ormotion in microelectromechanical systems (MEMS) devices. Such drivedevices are employed, for example, in the fabrication of MEMS-typeaccelerometers, gyroscopes, and inertia sensing devices where rapidactuation is often necessary to effectively measure and/or detect motionand acceleration.

In a typical comb drive device, a main body is supported over anunderlying support substrate using a number of anchors. One or moredrive elements electrically coupled to the main body can be actuated tomanipulate the main body above the support substrate in a particularmanner. In certain designs, for example, the drive elements may includea number of interdigitated comb fingers configured to convert electricalenergy into mechanical energy using electrostatic actuation.

One method of fabrication of electrostatic comb drive devices generallybegins with a silicon wafer substrate. A highly boron-doped layer isrealized through diffusion or epitaxial growth over the wafer substrate,which can then be etched to form the desired microstructures using apatterned mask layer and a suitable etch process, such as the Bosch-typeDeep Reactive Ion Etch (DRIE). The etched wafer is then bonded to anunderlying support substrate using a suitable bonding process such asanodic bonding. The support substrate may include a number of mesas thatsupport the main body and drive elements above the support substratewhile allowing movement thereon, and metal patterns appropriate forconnecting to the silicon members. One or more electrodes can also beprovided on the support substrate to measure up/down movement of themain body caused by, for example, acceleration or rotation of thesensor. The silicon substrate wafer is then removed through one or morenon-selective and selective etch processes, such as KOH and EDP basedetching, leaving only the patterned, highly doped silicon structure.

For the force of the comb drive to be applied uniformly as the devicemoves back and forth, the shape or profile of the etched structureshould be as uniform as possible. The uniformity of the etched structureis dependent on a number of factors including, for example, the gapbetween adjacent features, and the parameters of the DRIE process used.Since etching tends to be slower at those locations where there arerelatively small gaps between adjacent features, the profile of the combfingers tend to be non-uniform along their length, due to the varyinggap sizes caused by the partial overlap of the comb fingers. Thisnon-uniformity may result in changes in capacitance as the comb fingersmove with respect to each other, producing undesired electricalharmonics in the motor drive power. These additional harmonics canreduce the desired motive force of the comb fingers, resulting ingreater energy dissipation and noise in the sensor output. In somecases, the non-linear profile of the comb fingers may producequadrature, or motion out-of-plane, which further creates noise in thesensor output. As such, there is a need in the art for improvedfabrication methods for reducing harmonic distortion in comb drivedevices.

SUMMARY OF THE INVENTION

The present invention relates to fabrication methods for reducingharmonic distortion in comb drive devices. In an illustrative method ofthe present invention, an epitaxial layer of, for example, highlyboron-doped (p++) silicon or other suitable material can be grown orformed on the surface of a wafer substrate used to form an electrostaticcomb drive device. A patterned mask layer defining a number of combdrive elements and one or more sacrificial etch-buffers can be formed onthe p++layer. Etch-buffers can be employed to make the gap betweenadjacent features more uniform. These etch-buffers can be used at ornear those locations where producing a uniform etched profile isdesirable. In some cases, the etch-buffers can be relatively small insize, such as when positioned between adjacent fingers of a comb drivestructure. In other cases, the etch-buffers can be larger in size, suchas when it is also desirable to fill in area that does not need to beetched. The pattern can be etched into the p++ layer and through to theunderlying substrate.

In some cases, the relatively small etch-buffers that are placed betweenadjacent fingers of a comb drive structure may be difficult to removeduring final wafer dissolution. Therefore, and in accordance with someembodiments of the present invention, the relatively small etch-buffersare removed before wafer bonding, but this is not required. Thus, and insome illustrative embodiments, after patterning the p++ layer, the wafermay be immersed in a suitable etchant, such as EDP, and the relativelysmall etch-buffers are fully undercut and liberated through, forexample, a combination of etching, rinsing and cleaning. Because oftheir size, only the relatively small etch-buffers are fully undercut,and all of the desired features, as well as any larger etch-buffers ifpresent, remain attached to the silicon substrate.

The top of the etched wafer substrate can be bonded to an underlyingsupport substrate, using a suitable bonding process such as anodicbonding. Once bonded to the support substrate, a final backside etchingprocess may be performed to remove the remaining wafer substrate, usingthe boron-doped silicon layer as an etch stop. When the relatively smalletch-buffers are not removed before wafer bonding, the final backsideetching process will liberate the etch-buffers. In some cases, acombination of rinsing and cleaning may be used to help remove theetch-buffers from the resulting structure.

When employed, the sacrificial etch-buffers may act to reduce oreliminate the relatively large gaps between non-overlapping regions ofthe comb drive fingers. The etch-buffers can be formed at selectiveregions on the wafer substrate to ensure a relatively uniform etch ratealong the length and/or at the ends of the comb fingers. In certainembodiments, for example, the sacrificial etch-buffers can be used toprovide a uniform gap between the sides of adjacent comb fingers. Duringetching, this uniform gap may reduce differences in etch rates that canoccur along the length of the comb fingers, thus providing a moreuniform profile to the comb fingers. As a result, the capacitive forceinduced as the comb fingers move with respect to each other tends to bemore linear, which may substantially reduce the introduction ofelectrical harmonics into the drive system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an illustrative prior art electrostaticcomb drive device including a number of comb drive elements having anon-uniform profile;

FIG. 2 is a cross-sectional view showing the profile of one of the combfingers along line 2-2 in FIG. 1;

FIG. 3 is a schematic view of an electrostatic comb drive device inaccordance with an illustrative embodiment of the present invention;

FIG. 4 is a transverse cross-sectional view of one of the comb fingersalong line 4-4 in FIG. 3;

FIG. 5 is a longitudinal cross-sectional view of one of the comb fingersand sacrificial etch-buffers along line 5-5 in FIG. 3; and

FIGS. 6A-6H are schematic representations of an illustrative method offabricating a comb drive device utilizing one or more sacrificialetch-buffers.

DETAILED DESCRIPTION OF THE INVENTION

The following description should be read with reference to the drawings,in which like elements in different drawings are numbered in likefashion. The drawings, which are not necessarily to scale, depictselected embodiments and are not intended to limit the scope of theinvention. Although examples of construction, dimensions, and materialsare illustrated for the various elements, those skilled in the art willrecognize that many of the examples provided have suitable alternativesthat may be utilized.

FIG. 1 is a schematic view of an illustrative prior art electrostaticcomb drive device 10 including a number of comb drive elements having anon-uniform profile. Comb drive device 10, illustratively a linear combdrive device, includes first and second comb drive members 12,14 eachformed in an opposing manner over a glass support substrate 16. While aglass support substrate 16 is used, it is contemplated that the supportsubstrate 16 may be made from any suitable material or material system.

The first comb drive member 12 includes a number of comb fingers 18 thatare interdigitated with a number of comb fingers 20 coupled to thesecond comb drive member 14. In the particular view depicted in FIG. 1,only a portion of the first and second drive members 12,14 are shown forsake of clarity. It should be understood, however, that othercomponents, in addition to those specifically discussed herein, may bedisposed on or above the support substrate 16.

During electrostatic actuation, the first comb drive member 12 isconfigured to remain stationary above the support substrate 16. Thesecond comb drive member 14, in turn, is freely suspended above thesupport substrate 16, and is configured to move back and forth relativeto the first comb drive member 12. A suspended spring 22 may be providedbetween the second comb drive member 14 and an anchor 23, where theanchor is fixed to the support substrate 16. The suspended spring 22provides a restoring force to the second comb drive member 14 when thedrive voltage passes through zero.

An external AC voltage source (not shown) having leads connected to thefirst and second comb drive members 12,14 can be configured to deliver acharge to the first and second comb fingers 18,20, inducing motiontherebetween. The voltage source can be configured to output atime-varying voltage signal to alternate the charge delivered to thecomb fingers 18,20, which in conjunction with the suspended spring 22,causes the second drive member 14 to oscillate back and forth in aparticular manner relative to the first comb drive member 12.

As can be further seen in FIG. 1, each comb finger 18,20 extendslongitudinally from a base portion 24 to an end portion 26 thereof. Inthe illustrative prior art electrostatic comb drive device depicted inFIG. 1, the comb fingers 18,20 are aligned in a parallel manner, and areconfigured to move longitudinally relative to each other when energizedby the AC voltage source. An overlapping region 28 disposed between thesides 30 of each laterally adjacent comb finger 18,20 forms a relativelysmall gap 32 that is sufficiently small (e.g. 1 to 2 microns wide) toinduce a sufficient capacitance between the comb fingers 18,20. Arelatively large gap 34 (e.g. 7 to 9 microns wide), in turn, separatesthe non-overlapping regions between each comb finger 18,20. Duringactuation, movement of the second comb fingers 20 relative to the firstcomb fingers 18 causes the amount of overlap at the overlapping region28 to change over time. The relatively small gap 32 between eachlaterally adjacent comb finger 18,20 remains constant, however, based onthe longitudinal arrangement of the comb fingers 18,20.

Fabrication of the prior art electrostatic comb drive device istypically accomplished using a silicon wafer substrate that is etched toform the desired drive elements, and then bonded to the underlyingsupport substrate 16 by anodic bonding, adhesive, or other suitablebonding method. The gaps 32,34 separating the various comb fingers 18,20are typically formed with a plasma etch tool configured to run aBosch-type gas-switching Deep Reactive Ion Etch process (DRIE).

The efficacy of the DRIE process to form comb drive elements having auniform profile is dependent in part on the etch rate of the DRIEprocess. The etch rate is typically optimized to provide a uniformprofile along the smallest gap 32 between comb fingers 18,20. Asindicated by the dotted lines in FIG. 1, for example, an increasedamount of lateral etching typically occurs along the sides 30 of thecomb fingers 18,20 at those regions where the gap spacing betweenadjacent comb fingers 18,20 is greater than the smallest gap 32. Inaddition, an increased amount of lateral etching typically occurs acrossthe end portions 26 of each comb fingers 18,20, causing the end portions26 to have a non-uniform profile.

FIG. 2 is a transverse cross-sectional view showing the profile of oneof the comb fingers 20 along line 2-2 in FIG. 1. As shown in FIG. 2, thevertical etch profile of the comb finger 20 tends to be asymmetric, witha greater amount of etching occurring along the sides 30 towards thebottom surface 36 of the comb finger 20 than at the top surface 38thereof. As a result, the width W₁ at the bottom surface 36 of the combfinger 20 tends to be smaller than the width W₂ at the top surface 38.This undercutting of the comb finger 20 is due, in part, to the geometryand size of the trench. Higher DRIE etch rates remove more siliconduring each etch cycle, both vertically and laterally, since the etchphase is somewhat isotropic. As a result, the profile of the combfingers 18,20 tends to be more non-uniform along its length, having agenerally greater undercut profile at those regions where there is noimmediately adjacent structure to reduce the gap and thus the rate ofetching.

During operation, a time-varying electrical signal is applied across thecomb fingers 18,20, inducing an opposite charge along the length of eachlaterally adjacent comb finger 18,20. This time-varying charge generatesa motive force that causes the comb fingers 18,20 to move back and forthwith respect to each other. As the comb fingers 18,20 move with respectto each other, however, the non-uniform profile along the length of thecomb fingers 18,20 induces a non-linear change in capacitance, causingelectrical harmonic distortion to be introduced into the motor drivepower. This introduction of harmonics into the drive power may increasethe amount of energy required to electrostatically actuate the movingmember at the desired frequency, and increase the complexity of thedrive electronics necessary to control movement. In some circumstances,the non-uniform profile of the comb fingers 18,20 can cause quadrature,or motion out-of-plane, creating more noise in the sensor output.

Referring now to FIG. 3, an electrostatic comb drive device 40 inaccordance with an illustrative embodiment of the present invention isillustrated. Comb drive device 40, illustratively a linear-type combdrive device, includes a first comb drive member 42 and a second combdrive member 44, each formed in an opposing manner on top of anunderlying support substrate 46. The first comb drive member 42 caninclude a number of comb fingers 48 interdigitated with a number of combfingers 50 which are coupled to the second comb drive member 44.Although not shown in FIG. 3, a suspended spring may also be provided toproduce a restoring force when the drive voltage passes through zero,similar to that described above.

The comb fingers 48,50 can be configured to operate in a manner similarto the comb fingers 18,20 described above. In certain embodiments, forexample, a square wave, sinusoidal wave, or any other suitabletime-varying AC voltage signal can be applied to the comb fingers 48,50,causing the comb fingers 48,50 to move back and forth with respect toeach other in a desired manner above the underlying support substrate46. While a linear-type comb drive device is specifically illustrated inFIG. 3, it should be understood that the comb drive members 42,44 andassociated comb fingers 48,50 can be configured to move in some otherdesired fashion. For example, the comb drive device 40 may comprise arotary-type comb drive device having a configuration similar to thatdescribed in PCT International Application Number PCT/US01/26775, whichis incorporated herein by reference in its entirety.

To reduce harmonic distortion caused by non-uniformities in the profileof the comb fingers 48,50, comb drive device 40 may be formed using oneor more sacrificial etch-buffers. In the schematic view depicted in FIG.3, for example, a number of sacrificial etch-buffers 52 are shown atselective locations adjacent to the comb fingers 48,50. As is discussedin greater detail below with respect to FIGS. 6A-6H, these etch-buffers52 can be used during the fabrication process to minimize the relativelylarger gaps between adjacent structures that can cause non-uniformitiesin the profile of the comb fingers 48,50. As indicated by dashed lines,the sacrificial etch-buffers 52 are later removed in a etch-bufferremoval process, leaving intact a structure similar to that discussedabove with respect to FIG. 1.

In the illustrative embodiment, the sacrificial etch-buffers 52 may havea substantially rectangular shape defining first and second opposingsides 54, and first and second opposing ends 56. The specific dimensionsof the sacrificial etch-buffers 52 employed can vary depending on thedimensions and relative spacing of the comb drive fingers 48,50, and thedistance between the comb fingers 48,50 and other adjacent structuressuch as the leading ends 62,64 of each comb drive member 42,44. Thesacrificial etch-buffers 52 may be placed within the non-overlappingregions of the comb fingers 48,50, and can be dimensioned such that thefirst and second opposing sides 54 of the sacrificial etch-buffer 52 aresubstantially parallel and adjacent with the sides 58 of each laterallyadjacent comb finger 48,50.

A first narrowed gap 66 formed between one or both sides 54 of thesacrificial etch-buffers 52 can be utilized to match the etch rate alongthe sides 58 of the comb fingers 48,50 where no finger overlap exists.To provide a more uniform profile along the length of the comb fingers48,50, the first narrowed gap 66 may be made to approximate thefixed-width gap 66 that normally exists between the overlapping regionsof the comb fingers 48,50. In certain embodiments, for example, thefirst narrowed gap 66 can have a width of approximately 1 to 2 microns,similar to the width at gap 66. It should be understood, however, thatthe first narrowed gap 66 is not necessarily limited to such dimension.

The sacrificial etch-buffers 52 can also be dimensioned to form a secondnarrowed gap 70 located between the ends 58 of the comb fingers 48,50and the ends 56 of the sacrificial etch-buffers 52. The second narrowedgap 70 can be made to approximate the gap 66 that exists betweenoverlapping regions of the comb fingers 48,50. In certain embodiments,for example, the second narrowed gap 70 can have a width ofapproximately 1 to 2 microns, similar to the width at gap 66. As withthe first narrowed gap 66, the second narrowed gap 70 reduces thedifferential rate of etching that can occur across the ends 60 of thecomb fingers 48,50.

FIG. 4 is a transverse cross-sectional view of one of the comb fingers50 along line 4-4 in FIG. 3. As shown in FIG. 4, the profile of the combfinger 50 is substantially symmetric, with an equal amount of etchingoccurring at the bottom surface 72 of the comb finger 50 as at the topsurface 74. By employing sacrificial etch-buffers 52 adjacent to thesides 56 of the comb fingers 48,50, the effects of undercutting may bereduced and/or eliminated. As a result, the sides 56 of the etched combfingers 48,50 tend to be substantially vertical in orientation, with amore uniform width W across the entire thickness of the comb finger 50.

FIG. 5 is a longitudinal cross-sectional view showing the profile of oneof the comb drive fingers 50 and sacrificial etch-buffers 52 along line5-5 in FIG. 3. As shown in FIG. 5, the sacrificial etch-buffer 52 can bepositioned laterally adjacent and in-line with the comb finger 50 andleading end 62 of the first comb drive member 42. One end 56 of thesacrificial etch-buffer 52 can be spaced apart a distance from the end60 of the comb finger 50, forming the second narrowed gap 70 discussedabove. A similarly dimensioned third narrowed gap 76 disposed betweenthe leading end 60 of the first comb drive member 42 and the oppositeend 56 of the sacrificial etch-buffer 52 may also be provided to matchthe etch rate along the leading ends 60,62 of the comb drive member 42,if desired.

In certain embodiments, the second and third narrowed gaps 70,76 formedby the sacrificial etch-buffer 52 can be made to approximate the gap 66disposed between adjacent sides 58 of the comb fingers 48,50. In oneillustrative embodiment, for example the second and third narrowed gaps70,76 may have a width of approximately 1 to 2 microns, similar to thewidth provided at gap 66. By providing a constant gap width at theselocations, the etch rate at the ends 60 of the comb fingers 48,50 and atthe leading ends 62,64 of the comb drive members 42,44 can be furthercontrolled.

While the illustrative embodiment shown in FIGS. 3-5 depictsetch-buffers having a substantially rectangular shape along theirlength, it should be appreciated that the etch-buffers can assume otherdesired shapes depending on the particular type of comb drive deviceemployed. In a rotary-type comb drive device utilizing curved combfingers, for example, the sacrificial etch-buffers can be made to assumea curved shape along one or more of its sides to provide a uniform gapalong the length of the comb fingers. As with other embodiments herein,the sacrificial etch-buffers can be used to match the etch rate at thoseregions where there is no immediately adjacent structure, resulting inless undercutting of the comb fingers.

Operation of the comb drive device 40 is similar to that discussed abovewith respect to the aforesaid prior art electrostatic comb drive device10. A time-varying electrical signal can be applied to the comb fingers48,50, inducing the opposite charge along the length of each laterallyadjacent comb finger that moves the comb fingers 48,50 toward eachother. The electrical signal can be varied to oscillate the comb fingers48,50 back and forth relative to one another in a desired manner. Incertain embodiments, for example, an external AC voltage configured tooutput a square wave voltage signal can be utilized to reciprocate thecomb fingers 48,50 back and forth relative to each other. In otherembodiments, a sinusoidal wave may be used.

FIGS. 6A-6H are schematic representations of an illustrative method offabricating a comb drive device using one or more sacrificialetch-buffers. These drawings show one comb finger 112, one etch-buffer108, and one comb anchor 110 for clarity. It is implied that thecross-sections in 6A-6H represent a comb drive similar to FIG. 3.Beginning with FIG. 6A, a wafer 78 having a first surface 80 and asecond surface 82 is provided as a sacrificial substrate, which is laterremoved through an etching process. The wafer 78 can be formed from anynumber of suitable materials capable of being etched using semiconductorfabrication techniques such as micromasking. While silicon is typicallythe most common wafer material used, it will be appreciated by those ofskill in the art that other suitable materials such as gallium,arsenide, germanium, glass, or the like can also be used, if desired.

As shown in a second step in FIG. 6B, a layer 84 of boron-doped siliconcan be grown or formed on the first surface 80 of the wafer 78. In someembodiments, for example, building of the epitaxial layer 84 includes anepitaxially grown single-crystal silicon layer that is heavily dopedwith boron. Other dopants such as indium, thallium, and aluminum mayalso be used to form the epitaxial layer 84, if desired. In use, thedopant (e.g. boron) contained in the grown or formed epitaxial layer 84can be used as an etch stop in later fabrication steps to facilitateremoval of the wafer 78, leaving only the relatively thin epitaxiallayer 84 to form the various comb drive elements of the comb drivedevice.

In some cases, the relatively high concentration of dopant within thefirst epitaxial layer 84 can cause intrinsic tensile stresses within thewafer 78. These intrinsic tensile stresses can cause the wafer 78 to bowor cup enough to make processing of the wafer impractical. To minimizewafer bowing caused by the growth of the epitaxial layer 84 on only oneside of the wafer 78, and in an illustrative embodiment, a secondepitaxial layer 86 can be grown or formed on the second surface 82 ofthe wafer 78 to counterbalance the stresses imparted to the wafer 78 bythe first epitaxial layer 84. As shown in FIG. 6B, the resulting wafer78 substrate is sandwiched between opposing layers 84,86 of boron-dopedsilicon, forming a bottom side 90 and a top side 88 of the wafer 78.

FIG. 6C illustrates another fabrication step involving the use of aphotomask cap layer 92 on the top side 88 of the first epitaxial layer84. The cap layer 92 can be patterned using a suitable process such asphotolithography to form the desired elements of the comb drive device,as well as other desired components. A number of channels 94,96 exposingthe top side 88 of the first epitaxial layer 84 to the surroundingenvironment allow an etchant to flow downwardly through the firstepitaxial layer 84 and onto the first surface 80 of the wafer 78.

In the particular view depicted in FIG. 6C, the cap layer 92 includes afirst mask region 98 that can be used to define one of the sacrificialetch-buffers depicted, for example, in FIG. 3. The first masked region98 can be spaced apart from a second and third mask region 100,102 ofthe cap layer 92, which can be used, for example, in defining thestructure of each of the comb fingers 48,50 illustrated in FIG. 3.

FIG. 6D illustrates the step of etching the first epitaxial layer 84 andwafer substrate 78 to define the comb drive elements and etch-buffers ofthe comb drive device. Using a suitable etching process such as DeepReactive Ion Etching (DRIE), which relies on the gas composition in thesurrounding atmosphere and applied RF power, a number of trenches104,106 can be formed through the first epitaxial layer 84 and, in somecases, into the top surface 80 of the wafer 78. During the etchingprocess, the existence of the first mask region 98 above the firstepitaxial layer 84 prevents the removal of material immediately belowthe cap layer 92, forming an etch-buffer 108. In similar fashion, thesecond and third mask regions 100,102 prevent the removal of materialbelow the cap layer 92, forming a number of comb fingers 112, and combanchors 110, spaced apart from the etch-buffer by the trenches 104,106.

Because the etchant is typically optimized for the gap or spacingbetween adjacent fingers 48,50, as the etchant travels through thechannels 94,96 (FIG. 6C) to form the trenches 104,106, the existence ofthe etch-buffer 108 matches the width of the trench, and helps ensurethat the consumption of reactant species used in the DRIE process issubstantially uniform. As a result, the vertical etch profile of thetrenches 104,106 is substantially linear, with an equal amount ofetching occurring at the top of the trench 104,106 as at the bottom.Once the trenches 104,106 have been formed, the cap layer 92 is strippedoff in a manner leaving the etched first epitaxial layer 84 and wafer 78intact for further processing.

In some cases, the relatively small etch-buffers that are placed betweenadjacent fingers of the comb drive structure may be difficult to removeduring final wafer dissolution. Tests conducted without removing therelatively small comb etch-buffers prior to wafer bonding showed thatthey can become stuck in areas of the finished device, where they aredifficult, and in some cases, virtually impossible to remove. This issuegenerally does not affect larger etch-buffers because they are too bigto easily become trapped or lodged. Trapped etch-buffers can cause asubstantial yield loss. Therefore, and in accordance with someembodiments of the present invention, the relatively small combetch-buffers may be removed before wafer bonding, but this is notrequired.

FIG. 6E illustrates the optional step of removing the relatively smalletch-buffers 108 from the comb drive portion of the device prior towafer bonding. A selective etchant, such as the anisotropic,ethylenediamine-based etchant PSE 300-S (EDP-S) available from theTransene Company, of Danvers, Mass. can be used at the lower end of thetemperature range (i.e. at about 100° C.) for a suitable period (e.g. 20minutes) to completely undercut the relatively small etch-buffers,forming a shallow cavity 114 under the etch-buffers 108, and causingthem to be free to float away. During this process the comb fingers 112may also be completely undercut, but because they are attached to largerfeatures (42,44 of FIG. 3), they remain intact. While EDP is used in theillustrative embodiment, it should be understood that any suitableselective etching procedure may be used to undercut and liberate therelatively small etch-buffers.

In most cases, not all of the relatively small etch-buffers 108 willcompletely disengage from their position on the silicon wafer during theEDP etch process. Thus, at least in these cases, furtherremoval-enhancing steps may be desirable. Rinsing, such as overflowrinsing with de-ionized water (DI H2O), which is used to remove EDPresidue, will remove some of the liberated etch-buffers 108. Butcomplete removal may require a more aggressive cleaning process in mostcases.

In one illustrative removal step depicted in FIG. 6F, for example, anacoustical cleaning process may be used to help liberated theetch-buffers 108 from the surrounding structure. In the illustrativeembodiment, the formed comb drive device structure can be submersedwithin a bath 126 containing a suitable fluid such as de-ionized (DI)water. An acoustical source 124 (e.g. a piezoelectric transducer)capable of producing acoustical pressure waves can be activated withinthe bath 126 to acoustically clean the various structures of the combdrive device. As shown in FIG. 6F, for example, the acoustical energyemitted from the transducer 124 can be used to agitate the fluidsurrounding the etch-buffers 108, causing the etch-buffers 108 to floataway into the surrounding fluid. A DI water rinse and dry cycle mayfollow the cleaning step, if desired.

In certain embodiments, the acoustical source 124 can be configured toclean the structure and/or help liberate the etch-buffers 108 using amegasonic cleaning process, which utilizes relatively low energy sonicpressure waves in the range of about 400 kHz to 1.2 MHz. Similar toother cleaning techniques such as ultrasonic cleaning, megasoniccleaning relies on the principal of acoustical cavitation. In contrastto ultrasonic cleaning, however, megasonic cleaning producessignificantly smaller bubbles, resulting in a lower release of energy atimplosion, causing little or no damage to a surface subjected to thisprocess. In some embodiments, a solution of Summaclean SC-15 containinga number of surfactant additives can be added to the bath 126 to aid inthe removal of, and limit redeposition of the etch-buffers 108 withinthe structure. While megasonic cleaning is generally preferred for itsability to gently clean the structure, it should be understood thatother suitable cleaning processes could be utilized, if desired.

FIG. 6G illustrates the step of bonding the etched wafer 78 of FIG. 6Dto an underlying support substrate 122. In some illustrativeembodiments, the underlying support substrate 122 can be formed from asuitable dielectric material that can be used to electrically isolatethe various components of the comb drive device. In certain embodiments,for example, the underlying support substrate 122 may be formed ofsuitable glass material such as Pyrex® Corning Type No. 7740.

One or more metallic electrodes 120 and conductive traces (not shown)disposed above a top surface 118 of the underlying support substrate 122can be used to provide electrical connections to the various sensingelements of the comb drive device. The metallic electrodes 122 may beformed using techniques well known to those skilled in the art. Incertain embodiments, for example, the metallic electrodes 122 may beformed by sputtering or evaporating metallic particles (e.g. titanium,platinum, gold etc.) onto the top surface 118 using a suitablesputtering or evaporation processes, and photolithography and etch orlift-off techniques.

The underlying support substrate 122 may further include one or moremesas 116 extending upwardly from the top surface 118. The mesas 116 canbe formed by etching away a portion of the top surface 118 of theunderlying support substrate 122, leaving intact the material at themesas 116. Alternatively, the mesas 116 can be formed by building upmaterial from the top surface 118. In either embodiment, the mesas 116can be configured to support the comb drive members above the topsurface 118 in a manner that permits freedom of movement.

Once the electrodes, conductive traces, supports and other desiredelements have been formed on the underlying support substrate 122, theetched wafer 78 depicted generally in FIG. 6E is then flipped orinverted such that the topside 88 of the etched wafer 78 is positionedon top of the mesas 116 so as to overhang the top surface 118 of theunderlying support substrate 122. The etched wafer 78 and underlyingsupport substrate 122 are then bonded together using a suitable bondingprocess such as anodic bonding, wherein the two substrates are heated atan elevated temperature (e.g. 200-500° C.) and then chemically bondedtogether by applying a charge to the two members. Other suitable bondingprocesses such as heat bonding, adhesives, etc. may also be used to bondthe two members together, if desired.

FIG. 6H illustrates the final steps that can be used to fabricate thecomb drive device. As illustrated in FIG. 6H, the second epitaxial layer86 and the remaining portions of the etched wafer 78 are removed,leaving intact the heavily boron-doped microstructures overhanging thetop surface 118 of the underlying support substrate 122. The removal ofthe wafer 78 can be accomplished by using a combination of non-selectiveprocesses such as wafer grinding or KOH-based etching, and selectiveprocesses which will not attack or significantly attack the highly borondoped silicon. In certain embodiments, for example, the second epitaxiallayer and most of the substrate 78 can be removed using an industrystandard grinding process, leaving enough of the substrate 78 to not hitthe patterned epitaxial layer. The remaining silicon can then beselectively removed using an anisotropic, ethylenediamine-based etchantsuch as PSE 300-F available from the Transene Company, of Danvers, Mass.for a suitable time to remove any remaining material. It should beunderstood, however, that any number of suitable doping-selectiveetching procedures could be used to remove the remaining wafer material.

In some cases not all of the remaining etch-buffers (such as largeretch-buffers) will completely disengage from their position on thebonded wafer during the etch process and further removal-enhancing stepsmay be desirable. Rinsing, such as overflow rinsing with de-ionizedwater (DI H2O), which is used to remove EDP residue will remove some ofany liberated etch-buffers. But complete removal may require a moreaggressive cleaning process in some cases.

In one illustrative removal step similar to that depicted in FIG. 6F,for example, an acoustical cleaning process can be used to help removethe liberated etch-buffers from the surrounding structure. The bondedcomb drive device structure can be submersed within a bath 126containing a suitable fluid such as de-ionized (DI) water. An acousticalsource 124 (e.g. a piezoelectric transducer) capable of producingacoustical pressure waves can be activated within the bath 126 toacoustically clean the various structures of the comb drive device. Asshown in FIG. 6F, for example, the acoustical energy emitted from thetransducer 124 can be used to agitate the fluid surrounding theetch-buffers, causing the etch-buffers to float away into thesurrounding fluid. A DI water rinse and dry cycle may follow thecleaning step, if desired.

In certain embodiments, the acoustical source 124 can be configured toclean the structure using a megasonic cleaning process, which utilizesrelatively low energy, sonic pressure waves in the range of about 400kHz to 1.2 MHz. Similar to other cleaning techniques such as ultrasoniccleaning, megasonic cleaning relies on the principal of acousticalcavitation. In contrast to ultrasonic cleaning, however, megasoniccleaning produces significantly smaller bubbles, resulting in a lowerrelease of energy at implosion, causing little or no damage to a surfacesubjected to this process. In some embodiments, a solution of SummacleanSC-15 containing a number of surfactant additives can be added to thebath 126 to aid in the removal of, and limit redeposition of theetch-buffers similar to 108 within the structure. While megasoniccleaning is generally preferred for its ability to gently clean thestructure, it should be understood that other suitable cleaningprocesses could be utilized, if desired.

When the relatively small etch-buffers are not removed before waferbonding, the final backside etching process will liberate the relativelysmall etch-buffers, along with any larger etch-buffers if present. Asdescribed above, a combination of rinsing and cleaning may be used tohelp remove these etch-buffers from the resulting structure, if desired.

Having thus described the several embodiments of the present invention,those of skill in the art will readily appreciate that other embodimentsmay be made and used which fall within the scope of the claims attachedhereto. Numerous advantages of the invention covered by this documenthave been set forth in the foregoing description. It will be understoodthat this disclosure is, in many respects, only illustrative. Changesmay be made in details, particularly in matters of shape, size andarrangement of parts without exceeding the scope of the invention.

1. A comb drive device configured to produce less harmonic distortionduring operation, comprising: a first comb drive member including afirst plurality of comb fingers each having a base portion, an endportion, and a number of sides defining a length; and a second combdrive member including a second plurality of comb fingers each having abase portion, an end portion, and a number of sides defining a length,said second plurality of comb fingers being spaced apart from andinterdigitated with said first plurality of comb fingers; wherein saidfirst and second plurality comb fingers each have a uniform profilealong their length.
 2. The comb drive device of claim 1, wherein saidfirst and second plurality of comb fingers each include an end portionhaving a uniform profile.
 3. The comb drive device of claim 1, whereinsaid first and second comb drive members are formed by an etchingprocess utilizing one or more sacrificial etch-buffers.
 4. The combdrive device of claim 3, wherein said one or more sacrificialetch-buffers are placed at non-overlapping regions of the comb fingers.5. The comb drive device of claim 4, wherein each sacrificialetch-buffer forms a first narrowed gap along the length of each adjacentcomb finger.
 6. The comb drive device of claim 5, wherein said firstnarrowed gap is substantially equal to a gap disposed betweenoverlapping sides of each laterally adjacent comb finger.
 7. The combdrive device of claim 5, wherein each etch-buffer further forms a secondnarrowed gap at the end portion of each comb finger.
 8. The comb drivedevice of claim 1, further comprising an underlying support substrateadapted to freely support the first and second comb drive members. 9.The comb drive device of claim 1, wherein said comb drive device is anelectrostatic comb drive device.
 10. The comb drive device of claim 1,wherein said comb drive device is a linear-type comb drive device. 11.The comb drive device of claim 1, wherein said comb drive device is arotary-type comb drive device.
 12. A comb drive device configured toproduce less harmonic distortion during operation, comprising: a firstcomb drive member including a first plurality of comb fingers eachhaving a base portion, an end portion, and a number of sides defining alength; a second comb drive member including a second plurality of combfingers each having a base portion, an end portion, and a number ofsides defining a length, said second plurality of comb fingers beingspaced apart from and interdigitated with said first plurality of combfingers; and one or more sacrificial etch-buffers selectively placed atnon-overlapping regions between adjacent comb fingers.
 13. The combdrive device of claim 12, wherein said first and second plurality ofcomb fingers each include an end portion having a uniform profile. 14.The comb drive device of claim 12, wherein said one or more sacrificialetch-buffers are placed at non-overlapping regions of the comb fingers.15. The comb drive device of claim 14, wherein each sacrificialetch-buffer forms a first narrowed gap along the length of each adjacentcomb finger.
 16. The comb drive device of claim 15, wherein said firstnarrowed gap is substantially equal to a gap disposed betweenoverlapping sides of each laterally adjacent comb finger.
 17. The combdrive device of claim 15, wherein each etch-buffer further forms asecond narrowed gap at the end portion of each comb finger.
 18. The combdrive device of claim 12, further comprising an underlying supportsubstrate adapted to freely support the first and second comb drivemembers.
 19. The comb drive device of claim 12, wherein said comb drivedevice is an electrostatic comb drive device.
 20. The comb drive deviceof claim 12, wherein said comb drive device is a linear-type comb drivedevice.
 21. The comb drive device of claim 12, wherein said comb drivedevice is a rotary-type comb drive device.
 22. A method of fabricating acomb drive device utilizing one or more sacrificial etch-buffers,comprising the steps of: providing a substrate; and etching a patterninto the substrate to form one or more interdigitated comb fingershaving overlapping and non-overlapping regions, the pattern alsoincluding one or more etch-buffers in at least one non-overlappingregion.
 23. A method of fabricating a comb drive device utilizing one ormore sacrificial etch-buffers, comprising the steps of: providing awafer substrate having a first surface and a second surface; applying afirst epitaxial layer onto the first surface of the wafer substrate;etching a pattern into the first epitaxial layer and at least a portionof the wafer substrate to form one or more comb drive structures withone or more sacrificial etch-buffers therebetween; liberating one ormore of the sacrificial etch-buffers from the comb drive structure witha selective etch; bonding the etched wafer substrate to an underlyingsupport substrate; and etching away the wafer substrate.
 24. The methodof claim 23, wherein the sacrificial etch-buffers are liberated using anEDP etchant.
 25. The method of claim 24, wherein said step of liberatingthe one or more sacrificial etch-buffers also includes an acousticalcleaning process.
 26. The method of claim 24, wherein said step ofliberating the one or more sacrificial etch-buffers also includes amegasonic cleaning process.
 27. The method of claim 24, furthercomprising the step of applying a second epitaxial layer onto the secondsurface of the wafer substrate prior to said step of etching a patterninto the first epitaxial layer and at least a portion of the wafersubstrate.
 28. The method of claim 24, wherein said step of bonding theetched wafer substrate to an underlying support substrate is performedby an anodic bonding process.
 29. The method of claim 24, furthercomprising the step of forming one or more mesas on a top surface of theunderlying support substrate, said one or more mesas adapted to freelysupport the etched comb drive structures thereabove.
 30. The method ofclaim 24, wherein said step of etching a pattern into the firstepitaxial layer and at least a portion of the wafer substrate includesthe step of applying a cap layer onto the a top side of the firstepitaxial layer, said cap layer defining a pattern of microstructuresincluding at least one sacrificial etch-buffer.
 31. The method of claim24, wherein said step of etching the first epitaxial layer and at leasta portion of the wafer substrate includes a deep reactive ion etch(DRIE) process.
 32. The method of claim 24, wherein said step of etchingthe first epitaxial layer and at least a portion of the wafer substrateis performed using photolithography.
 33. The method of claim 24, whereineach of said one or more sacrificial etch-buffers is configured toprovide a uniform gap width between adjacent comb drive structures. 34.A method of fabricating a comb drive device utilizing one or moresacrificial etch-buffers, comprising the steps of: providing a wafersubstrate having a first surface and a second surface; applying a firstepitaxial layer onto the first surface of the wafer substrate; applyinga cap layer onto the first epitaxial layer, said cap layer defining apattern of a microstructure including at least one sacrificialetch-buffer; etching the first epitaxial layer and at least a portion ofthe wafer substrate to form the microstructure with one or moresacrificial etch-buffers; removing the cap layer from the firstepitaxial layer; liberating one or more of the sacrificial etch-buffersfrom the microstructure with a selective etch process; bonding theetched wafer substrate to an underlying support substrate; and etchingaway the wafer substrate.
 35. The method of claim 34, wherein said stepof liberating one or more of the sacrificial etch-buffers also includesan acoustical cleaning process.
 36. The method of claim 34, wherein saidstep of liberating the one or more sacrificial etch-buffers alsoincludes a megasonic cleaning process.
 37. The method of claim 34,further comprising the step of applying a second epitaxial layer ontothe second surface of the wafer substrate prior to said step of etchinga pattern into the first epitaxial layer and at least a portion of thewafer substrate.
 38. The method of claim 34, wherein said step ofbonding the etched wafer substrate to an underlying support substrate isperformed by an anodic bonding process.
 39. The method of claim 34,further comprising the step of forming one or more mesas on a topsurface of the underlying support substrate, said one or more mesasadapted to freely support the etched microstructure thereabove.
 40. Themethod of claim 34, wherein said microstructure includes one or morecomb drive structures, and wherein at least one of said one or moresacrificial etch-buffers is configured to provide a uniform gap widthbetween adjacent comb drive structures.
 41. A method of fabricating acomb drive device utilizing one or more sacrificial etch-buffers,comprising the steps of: providing a wafer substrate having a firstsurface and a second surface; applying a first epitaxial layer onto thefirst surface of the wafer substrate; etching a pattern into the firstepitaxial layer and at least a portion of the wafer substrate to formone or more comb drive structures with one or more sacrificialetch-buffers therebetween; liberating one or more of the sacrificialetch-buffers from the comb drive structure with a selective etchprocess; acoustically removing the liberated etch-buffers; bonding theetched wafer substrate to an underlying support substrate; etching awaythe wafer substrate; and acoustically removing one or more of theliberated etch-buffers, if any.
 42. A method of fabricating a comb drivedevice utilizing one or more sacrificial etch-buffers, comprising thesteps of: providing a wafer substrate having a first surface and asecond surface; applying a first epitaxial layer onto the first surfaceof the wafer substrate; applying a second epitaxial layer onto thesecond surface of the wafer substrate; etching a pattern into the firstepitaxial layer and at least a portion of the wafer substrate to formone or more comb drive structures with one or more sacrificialetch-buffers therebetween; liberating one or more of the smallsacrificial etch-buffers from the comb drive structure with a selectiveetch process; bonding the etched wafer substrate to an underlyingsupport substrate; and etching away the wafer substrate.
 43. A method offabricating a comb drive device utilizing one or more sacrificialetch-buffers, comprising the steps of: providing a wafer substratehaving a first surface and a second surface; applying a first epitaxiallayer onto the first surface of the wafer substrate; etching a patterninto the first epitaxial layer and at least a portion of the wafersubstrate to form one or more comb drive structures with one or moresacrificial etch-buffers therebetween; bonding the etched wafersubstrate to an underlying support substrate; and etching away the wafersubstrate, thereby liberating said one or more of the sacrificialetch-buffers from the wafer substrate.